In recent years, in the field of automobiles, security robots, or the like, demand for precise detection of an obstacle in a moving direction has been increased for collision prevention. Furthermore, in infrastructures such as roads or railroads, demand for detection of an obstacle such as a vehicle, human, or an animal has also arisen for prevention of accident and smooth traffic. As a technique for detecting the obstacle, there is known a laser radar being a distance measurement device using optical scanning. In general, the laser radar emits a laser beam to measure a distance to the obstacle based on a time required for detecting reflected light.
As a general laser beam scanning method, there is known a technique for emitting a laser beam to a mirror or a polygon mirror having a plurality of mirror surfaces to scan the laser beam while rotating the polygon mirror.
In particular, the laser radar needs to detect a wide range, and thus a laser beam is required to be emitted to a wide range. An emission range in a scanning angle direction is determined based on a scanning angle and a spot size (cross-sectional size of beam emitted), but the scanning angle direction of the laser beam corresponds to a rotation direction of the mirror, so that the spot size does not need to be increased in the scanning angle direction, and a smaller spot size in the scanning angle direction is preferred, for increased resolution. In contrast, an emission range in a sub-scanning angle direction orthogonal to the scanning angle direction is determined based on the number of scan lines and a spot size of beam emitted (or angle of view of light receiving lens). However, since the number of scan lines has a limitation, the spot size of beam emitted is increased in the sub-scanning angle direction at the center of the scanning angle, and the spot size is often different between a horizontal direction and a vertical direction.
Patent Literature 1 discloses a configuration in which a rotation mirror has first reflecting surfaces and second reflecting surfaces, formed to have an angle of 90° between the first and second reflecting surfaces, beam emitted from a light source along a direction orthogonal to a rotation shaft is scanned after being reflected twice on the first and second reflecting surfaces, so that tilting of the rotation shaft due to rotational deviation may not cause distortion of scan line. Such a mirror configuration may be applied to the laser radar.